This is most likely a permissions / ownership issue for the session save path for php. This happened to me when I changed the User and Group in httpd.conf (apache) to a different username. The php sessions files must have their group id s changed at the same time. For Fedora, modify the group as below (substituting the group name of apache httpd):
sudo chgrp NEWUSER /var/lib/php/*
Restart httpd after making this change.
If this change does not work for your configuration, check your apache error logs (/var/log/httpd/error_log) for more information- they will give you the location of the session.save_path that needs your attention. For example:
Unknown: Failed to write session data (files). Please verify that the current setting of session.save_path is correct (/var/lib/php/session)